1. Field of the Invention
The present invention relates to a projection optical system for use in a projection type image display apparatus such as a projector.
2. Description of the Related Art
Conventionally, various passive-type projectors have been proposed in which an image forming element such as a liquid crystal display panel is illuminated with luminous flux from a light source and modulates the luminous flux into transmitted light or reflected light for displaying an image before the light is enlargingly projected onto a projection surface such as a screen by a projection optical system.
Some of the projection optical systems for use in such projectors allow projection in a direction which is oblique (diagonal) to the screen (hereinafter referred to as “oblique projection”) in order to achieve a larger size of a projected image and a smaller thickness of the apparatus at the same time.
For example, Japanese Patent Application Laid-Open No. H05(1993)-100312 proposes the oblique projection by using a wide-angle lens with a wide field angle as a projection optical system, disposing an image forming element and a screen with a displacement from the optical axis of the projection optical system to use marginal portions of the field angle.
In addition, Japanese Patent Application Laid-Open No. H05(1993)-80418 proposes the oblique projection by forming an intermediate image of image light from a light bulb by a first projection optical system and enlargingly projecting the intermediate image onto a screen by a second projection optical system in which the optical axis of each projection optical system is appropriately inclined.
In recent years, especially, projection optical systems which employ an off-axial (non-coaxial) optical system to realize the oblique projection have been proposed.
The off-axial optical system in the projectors means an optical system which, when a central principal ray is defined as light which passes from the center of an original image displayed on the image forming element to the center of an image (a finally formed image) projected onto the screen through the center of a pupil (an aperture stop), includes a curved surface to which the normal line at the intersection of the central principal ray and the surface is not on the optical path of the central principal ray. The reference axis of the off-axial optical system is shaped to have turns.
Since the off-axial optical system allows relatively free bending or routing of the optical path, a smaller optical system is easily formed. The constituent surfaces are typically asymmetric and aspheric, so that sufficient correction of aberration can be made in the oblique projection.
The usefulness thereof is described in Japanese Patent Application Laid-Open No. 2001-255462, Japanese Patent Application Laid-Open No. 2001-215412, Japanese Patent Application Laid-Open No. 2000-027307 and the like.
FIG. 9 shows a projection optical system proposed in Japanese Patent Laid-Open No. 2001-255462. In FIG. 9, L shows an illumination optical system, P shows an image display panel, and K shows a projection optical system which includes a plurality of reflecting surfaces configured by using the off-axial system for guiding light modulated by the image display panel P to a screen S to form an image on the screen S.
FIG. 10 shows a projection optical system proposed in Japanese Patent Application Laid-Open No. 2001-215412. In FIG. 10, P shows an image display panel and S shows a screen. The projection optical system includes a decentered refractive lens unit and an off-axial optical system including a plurality of reflecting surfaces.
To provide a larger projected image in the projector, a conceivable approach is to increase the projection distance or reduce the focal length to provide a wider field angle. Since the projection optical system proposed in Japanese Patent Application Laid-Open No. 2001-255462 shown in FIG. 9 forms an image of a pupil, it has a small focal length.
The position where the pupil image is formed, however, is located between the screen S and one of the reflecting surfaces having a curvature through which luminous flux from the image display panel P to the screen S passes last, so that it is necessary to ensure a sufficient distance from the position of pupil image formation to the screen. As a result, the projection optical system has a long projection distance.
On the other hand, in the projection optical system proposed in Japanese Patent Application Laid-Open No. 2001-215412 shown in FIG. 10, the ratio of the length of the central principal ray traveling from the screen S to the optical surface having an optical power closest to the screen S, to the length of the central principal ray traveling from the screen S to the image display panel P is set in a certain range. This prevents the projection distance and the diameter of an optical element closer to the screen S from increasing unacceptably.
In the range of conditions disclosed in Japanese Patent Application Laid-Open No. 2001-215412, however, it is difficult to sufficiently reduce the diameter of an optical system when the projection optical system enlargingly projects light onto a large screen with a shorter focal length (that is, the projection optical system supports a wider field angle).
The diameter of an optical system tends to increase in the region from the pupil surface to the screen. This is because the region from the image display panel to the pupil surface corresponds to the focal length in which a number of lenses cannot be disposed, so that lenses are placed necessarily between the pupil surface and the screen. In addition, especially, the diameter of an optical system located farthest from the pupil significantly increases. Thus, to prevent a considerable increase in the diameter of an optical system, the distance from the pupil surface to the reflecting surface farthest therefrom needs to be defined.